1. Field of the Invention
The present invention generally relates to a ringer unit driving system in a subscriber transmission system of a communications network, and more particularly to a ringer unit driving system provided in a channel unit provided between an exchange and a subscriber terminal, such as a telephone set.
2. Description of the Prior Art
FIG. 1 illustrates a subscriber transmission system, which includes an exchange EX, a remote terminal RT, a distance terminal DT, and telephone sets 1. The exchange EX, which is connected to networks, includes a switch SW, a central office terminal COT, and ringer units 2. The central office terminal COT converts analog signals from the switch SW into digital signals, and multiplexes these digital signals. A multiplexed digital signal is sent to a coaxial cable CO, which connects the central office terminal COT and the remote terminal RT. Further, the central office terminal COT receives a multiplexed digital signal from the coaxial cable CO and demultiplexes it into digital signals. Then, the central office terminal COT converts the digital signals into analog signals, which are sent to the switch SW. Telephone sets 1 are connected to the switch SW via subscriber lines VF (only one telephone set is illustrated for the sake of simplicity). One ringer unit 2 is connected to the switch SW.
The remote terminal RT, connected to the exchange EX via the coaxial cable CO, includes channel units CU, which have the functions of demultiplexing the received multiplexed signal into digital signals and converting the digital signals into analog signals to be sent to the telephone sets 1 connected to the remote terminal RT via the subscriber lines VF (only telephone set 1 is illustrated for the sake of simplicity). The channel units CU also have the functions of converting analog signals received from telephone sets 1 connected thereto into digital signals and multiplexing these digital signals and digital signals received from an optical fiber cable OF into a multiplexed digital signal. The remote terminal RT has the function of converting optical signals into electric signals and converting electric signals into optical signals. One ringer unit 2 is connected to the remote terminal RT and is provided in common to the channel units CU of the remote terminal RT.
The distance terminal DT includes a plurality of channel units CU, which are respectively connected to a plurality of telephone sets 1 (only one telephone set 1 is illustrated for the sake of simplicity). One ringer unit 2 is connected to the distance terminal DT and is provided in common to the channel units CU of the distance terminal DT. The channel units CU have the functions of demultiplexing the multiplexed digital signal from the optical fiber into digital signals and converting the digital signals into analog signals to be sent to the telephone sets 1. The channel units CU also have the functions of converting analog signals from the telephone sets 1 into digital signals and multiplexing these digital signals into a multiplexed digital signal. The distance terminal DT has the function of converting electric signals into optical signals and converting optical signals into electric signals.
When the telephone set 1 connected to the switch SW calls the telephone set 1 connected to the remote terminal RT, a ringing signal generated by the ringer unit 2 connected to the switch SW is sent to the central office terminal COT, which converts the ringing signal into a signaling signal (signaling bit). The channel unit CU in the remote terminal RT connected to the called telephone set 1 detects the signaling bit from the exchange EX, and drives the ringer unit 2 connected to the remote terminal RT.
FIG. 2 is a block diagram of channel units CU#1-CU#N, provided in either the remote terminal RT or the distance terminals DT, and the corresponding ringer unit 2. The channel unit CU#1 includes a D/A converter 11, a signaling detector 12, a switch driver 14 and a switch 14. The D/A converter 11 converts the received digital signal into an analog signal to be sent to the telephone set 1. The signaling detector 12 detects the signaling bit contained in the received digital signal. The switch driver 13 turns ON the switch 14 when the signaling bit is detected. The other channel units have the same configuration as the channel unit CU#1.
The ringer unit 2 includes a ringer 2a and a ringer driving source 2b. The output terminal of the ringer unit 2 is connected in common to the switch 14 of the channel units CU#1-CU#N.
The exchange EX drives the bell of the called telephone set 1 connected to the channel unit CU#1 in the following manner. The switch SW outputs the ringing signal (20 Hz, for example) generated by the ringer unit 2 to the central office terminal COT, which converts the ringer signal into the signaling bit. The signaling detector 12 of the channel unit CU#1 detects the signaling bit contained in the received digital signal, and sends a detection signal to the switch driver 13. In response to the detection signal, the switch driver 13 turns ON the switch 14, so the ringing signal generated by the ringer 2a is sent to the telephone set 1 via the switch 14.
The ringer driving source 2b in the remote terminal RT or the distance terminal DT obtains energy from a commercial source. Alternatively, the ringer driving source 2b in the distance terminal DT is a power source driven by the power source provided in the remote terminal RT. In this case, an additional cable is provided between the remote terminal RT and the distance terminal DT, and energy generated by the power source in the remote terminal RT is transferred to the distance terminal via the additional cable.
As shown in FIG. 2, the ringer driving source 2b is permanently connected to the ringer 2a. If the ringer unit 2 is often connected to one of the channel units CU#1-CU#N, the ringer driving source 2b is efficiently used. However, if the ringer unit 2 is not often connected to one of the channel units CU#1-CU#N, the ringer 2a wastefully consumes energy supplied by the ringer driving unit 2b. Normally, the remote terminal RT or the distance terminal DT accommodates a smaller number of terminals than the exchange EX, and the time for which the ringer unit 2 is not needed in the RT or DT will be much longer than the time for which the ringer is not needed in the exchange EX. Hence, a larger quantity of power is wasted in the remote terminal RT or the distance terminal DT than that in the exchange EX.
In order to reduce energy wasted in the ringer unit, Japanese Laid-Open Patent Publication No. 64-39188 discloses a radio telephone system in which an amplifier of a ringer generating circuit provided in a slave station is turned ON only when the slave station receives a ringer driving signal from a ground (master) station. However, the ringer itself (which corresponds to the ringer 2a shown in FIG. 2) is always supplied with energy from the ringer driving source. Hence, the ringer consumes energy even when the ringer generating circuit is maintained in the inactive state in which the ringer is not connected to any of the telephone sets.
The above problem will be serious when the ringer driving source 2b fails to supply energy due to a failure. In this case, a backup battery (not shown in FIG. 2) is connected to the ringer 2a instead of the ringer driving source 2b. In this case, the backup battery is always connected irrespective of whether or not the ringer unit 2 is needed, and the ringer 2b wastes energy generated by the backup battery.